1. Field of the Invention
The present invention relates to a telephone system which allows a hold releasing request from a central office line when the central office line is put in its holding state.
2. Description of the Related Art
Referring to FIG. 9, there is shown a prior art telephone system having a holding function which includes a key telephone set (EKT) 1, a central office line interface circuit unit (COU) 3, an EKT interface circuit unit (STU) 5, a time-division switch circuit unit (TSU) 7, a central control circuit unit (CCU) 9, a power supply circuit unit (PSU) 11, a data highway 13, a power supply bus 15, a pulse code modulation highway (PCM highway) 17, a central office line 19, and an AC plug 21.
The COU 3 comprises a central office line interface circuit 23, a transmission transformer 25, a hybrid circuit 27, a codec 29, a control circuit 31, and a voltage stabilizer circuit 33. The central office interface circuit 23 has a dial-pulse sending circuit, a hook switch circuit, a ringer detecting circuit, an inverted-polarity detecting circuit, a loop current circuit and a loop current detecting circuit (these circuits are not shown in the drawing). The transmission transformer 25 is used to block DC components of a signal received from the central office line 19. The hybrid circuit 27 separates a signal to be transmitted to the central office line 19 from a signal to be received therefrom. The codec 29 converts to an analog signal a transmitting digital signal received from the PCM highway 17 (DA conversion) to transmit the analog signal to the line 19, and also to converts an analog signal received from the line 19 to a digital signal (AD conversion) and then multiplex and sends it to the PCM highway 17. The control circuit 31 receives control data from the central control circuit 61 through the data highway 13 to control the central office interface circuit 23 and also specifies the time slot of the codec 29. The control circuit 31 also receives input data sent from the central office interface circuit 23 and applies it to the central circuit 61 via the data highway 13. The voltage stabilizer circuits 33, 35, 37 and 39 are used as power supplies of circuits mounted on the respective unit boards to receive and stabilize a voltage generated by the power supply circuit 63 through the power supply bus 15, respectively.
The ETK interface circuit unit (STU) 5 is a unit board for interface with the EKT 1. The STU 5 comprises transmission transformers 41 and 43, a power supply circuit or battery 45, hybrid circuits 47 and 49, a codec 51, a control circuit 53 and a voltage stabilizer circuit 35. The transmission transformer 41 is provided to transmit a voice signal to the EKT 1, while the transmission transformer 43 transmits control data to the EKT 1. The battery 45 supplies power to the EKT 1. The hybrid circuits 47 and 49 are used to separate a transmitting signal for a voice signal of the EKT 1 from a receiving signal, and to separate the upstream control data from the downstream control data. The control circuit 53 controls the input and output of the control data to the EKT 1 and specifies the time slot of the codec 51.
The time-division switch circuit unit (TSU) 7 comprises a digital time-division switch 55, a sound source circuit 57, a control circuit 59 and a voltage stabilizer circuit 37. The digital time-division switch 55 performs exchanges between channels in a PCM multiplexed state.
The sound source circuit 57 generates all tones necessary for the digital key telephone system such as a dial tone, a busy tone, a holding tone and so on, and applies these tones to the digital time-division switch 55 in the form of multiplexed data signals respectively. The control circuit 59 controls the switch 55 and the sound source circuit 57.
The central control circuit unit (CCU) 9 comprises a central control circuit 61 and a voltage stabilizer circuit 39. The circuit 61 controls the whole telephone system.
The power supply circuit unit (PSU) 11 comprises a power supply circuit 63 which is connected to an AC power source through the plug 21 attached thereto to supply a voltage to the respective units.
The operation of the telephone system will next be explained.
Now suppose that the system received an incoming signal from the central office line 19. In this case, the in-coming signal from the line 19 is detected by the central office line interface circuit 23 which in turn applies the information of the signal to the control circuit 31. The circuit 31 converts the information to a multiplexed signal and applies it to the central control circuit 61 via the data highway 13. The central control circuit 61 recognizes the received information as the incoming signal from the line 19 and transmits the incoming information to the control circuit 53 of the STU 5 and the control circuit 59 of the TSU 7. The control circuit 59, when receiving the information, controls the sound source circuit 57 to generate a ringing signal from the circuit 57. The circuit 59 also controls the digital time-division switch 55 to cause the ringing signal output data of the sound source circuit 57 to be carried on one of the channels of the PCM highway 17 associated with the time slot in synchronism with the STU 5. On the other hand, the control circuit 53 sends the control data to the EKT 1 through the hybrid circuit 49 and the transmission transformer 43 to activate an incoming circuit of the EKT 1. The codec 51 receives the ringing signal from the digital time-division switch 55, converts it from digital to analogy and sends it to the EKT through the hybrid circuit 47 and the transmission transformer 41, in synchronism with the time slot of the STU 5. The EKT 1 passes the ringing signal through a converter (not shown) provided therein and outputs the signal in the form of sound. As a result, the EKT 1 is informed that there is an incoming signal from the line 19. When the EKT answers, to it , it sends answer information to the control circuit 53 through the transmission transformer 43 and the hybrid circuit 49. The information is received by the central control circuit 61 through the data highway 13, in which case the circuit 61 in turn recognizes that the EKT 1 has answered, sends the information to the control circuit 31 through the data highway 13. The control circuit 31, upon receiving the information, closes the loop current circuit within the central office line interface circuit 23. Simultaneously with it, the central control circuit 61 also sends the EKT answer information to the control circuit 59 which in turn stops the ringing signal of the sound source circuit 57 and controls the digital timedivision n switch 55 to close the central office line 19 and the talking path or channel of the EKT 1, thus allowing the EKT 1 to be put in its talking mode with the line 19.
Now when the EKT 1 is subjected to a holding operation, the EKT 1 transmits the information indicative of the holding state to the central control circuit 61 through the hybrid circuit 49, the control circuit 53 and the data highway 13, at which time the circuit 61 recognizes the holding state and sends the information to the control circuit 59 through the data highway 13. The control circuit 59 controls the digital time-division switch 55 and the sound source circuit 57 to cause the sound source circuit 57 to send a holding tone in place of the transmitting signal of the EKT 1 to the line 19 through the digital time-division switch 55, the PCM highway 17, the codec 29, the hybrid circuit 27, the transmission transformer 25 and the line interface circuit 23, whereby the system is put in its hold state.
When the EKT 1 is subjected to a predetermined operation to release the hold state, a hold releasing signal is sent from the EKT 1 to the central control circuit 61 through the transmission transformer 43, the hybrid circuit 49, the control circuit 53 and the data highway 13, in which case the circuit 61 recognizes the signal information and transmits it to the control circuit 59. The circuit 59, when receiving the information, controls the digital time-division switch 55 and the sound source circuit 57 to send the transmitting signal in place of the holding tone from the EKT 1 through the switch 55, the PCM highway 17, the codec 29, the hybrid circuit 27, the transmission transformer 25 and the line interface circuit 23 to the line 19. As a result, the system is put again in the talking state with the line 19.
When the EKT 1 is subjected to an on-hook operation, a signal indicative of the on-hook is transmitted from the EKT 1 to the central control circuit 61 through the transmission transformer 43, the hybrid circuit 49, the control circuit 53 and the data highway 13. The circuit 61 recognizes the signal as the end of the speech and transmits it to the control circuits 31 and 59 via the data highway 13. The circuit 31, when receiving the signal from the circuit 61, controls the line interface circuit 23 to restore the COU 3 again to the stand-by state. This causes the control circuit 59 to put the digital time-division switch 55 in the stand-by state.
Next, to the central office line 19 from the EKT 1 will be considered. When the EKT 1 is subjected to a line-access keying operation, a line access signal is applied to the control circuit 53 through the transmission transformer 43 and the hybrid circuit 49. In circuit 53 the access signal is multiplexed and transmitted to the central control circuit 61 via the data highway 13. The circuit 61 recognizes the signal as an access signal to the line 19 and applies it to the control circuit 31 of the COU 3 through the data highway 13. This causes the circuit 31 to control the line interface circuit 23 to close the loop circuit.
A dialing operation of the EKT 1 causes a dialed signal to be inputted to the central control circuit 61 through the transmission transformer 43, the hybrid circuit 49, the control circuit 53 and the data highway 13. If the dialed signal corresponds to a dial pulse (DP) signal, then the dial-pulse sending circuit within the line interface circuit 23 sends the DP signal under control of the control circuit 31. When the dialed signal corresponds to a dual-tone multi-frequency (DTMF) signal, the control circuit 59 controls the sound source circuit 57 to send the DTMF signal to the line 19 through the sound source circuit 57, the digital time-division switch 55, the PCM highway 17, the codec 29, the hybrid circuit 27, the transmission transformer 25 and the line interface circuit 23.
However, the prior art telephone system has a problem that when the central office line 19 is put in the holding state from the side of the EKT 1, the speaker cannot talk with the party until such a holding state is released from the side of the EKT 1. Furthermore, it is impossible to release the holding state from the side of the line.